Chem Ch.2 Independent Qs, Q #4

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Chem Ch.2 Independent Qs, Q #4

Post by jbrostrom » Thu Nov 07, 2019 1:54 pm

I am confused about the answer to question 4. I have attached images of the problem below. I am confused because I thought that lone pairs were stored in hybridized orbitals, which would make the answer to the question 3, not 0. I have also attached an image from my school ochem text book which shows where my confusion stems from.
Let me know if anyone can clarify this for me!
IMG_4631 (3).jpg
Ch2, Q4
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IMG_4630 (4).jpg
Ochem Book
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Re: Chem Ch.2 Independent Qs, Q #4

Post by NS_Tutor_Mathias » Thu Nov 07, 2019 5:47 pm

Both sp2 and sp3 are hybridized orbitals, so you do not need to worry about that distinction as far as where lone pairs can be. They can be in sp2 hybridized orbitals just as well!

What this question is really driving at (in a slightly mean way) is the idea that resonance forms will determine the hybridization of a particular stereocenter, and that the structure as it is drawn can trick you into ignoring its aromatic nature. While there are slightly too many resonance hybrids of guanine to draw out in a reasonable amount of time, the important takeaway is that even the lone pairs on the nitrogens that you labeled as sp3 hybridized can participate in resonance. Therefore those nitrogens must actually be sp2 hyrbidized.

If you'd like a slower, less chaotic introduction to this idea: Consider whether histidine is aromatic. And from there determine whether one or both of the nitrogens in the imidazole ring of histidine are sp2 hyrbidized.
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